There are flow metering applications where conventional metering devices, such as turbine and other inferential types, nutating disk and other positive displacement types, electromagnetic, acoustic, and pressure differential types, etc., are not used because of their inability to effectively respond to extremely low flow rates. Such extremely low flow rates are often less then 100 feet per hour, and are typically in the range of 0.1 to 10 feet per hour. These application examples include groundwater measurement as related to pollution monitoring and salt water intrusion, and aquaculture as related to the movement and distribution of food, waste and water circulation.
One form of currently available flow detector utilizes a heated temperature sensing element, such as a thermistor, whereby the thermistor heated by an electric current is cooled by fluid flow. The variation in flow rate influences the thermistor temperature and its voltage drop. While such a detector can respond to extremely low flow rates, the instability of its output signal because of the thermistor resistance or electronic circuit drifts, fluid temperature changes, and the buildup of scale (film or debris on the surface of the sensor acting as a thermo-insulator to the fluid) requires that it be constantly calibrated or corrected, and severely limits its usefulness. However, if another thermistor, similar to the first one is added to detect fluid temperature and thereby provide a means, for example, in a bridge circuit for drift compensation due to fluid temperature changes, the overall sensor drift will be substantially reduced. This arrangement is often used for gas flow measurement. However, even with the temperature compensated thermistor sensor, there are drifts and uncertainties which limit the practical usefulness of such instruments to a relatively high flow rate.
The prior art is exemplified by Piety U.S. Pat. No. 2,620,658; Gompper U.S. Pat. No. 3,060,746; Cordell et al. U.S. Pat. No. 3,217,546; Thompson U.S. Pat. No. 3,433,078 and Kerfoot et al. U.S. Pat. No. 4,391,137.
The present invention eliminates the disadvantages and shortcomings attendant the conventional prior art devices and techniques, and at the same time provides a device and method which permit accurate measurements of extremely low flow rates of fluids with a minimum of parts and an extremely surprisingly reduced cost of manufacture.